Answer:
A. by the definition of perpendicular.
Step-by-step explanation:
Indeed, in geometry, for lines to be termed as been perpendicular, it means that two lines met at a right angle (90 degrees).
From the attached image we could see that Segment PR is perpendicular to segment QS as it touches or meets with segment QS which follows the definition of perpendicular. Also, Angle QRP is a right angle because it is exactly 90 degrees.